1. Field of the Invention
The present invention relates to a fixing apparatus frequently used in an image forming apparatus such as a printer, a copying machine or a facsimile system so as to fix a developed medium on a support medium.
2. Description of the Prior Art
Most of the conventional fixing apparatuses employ a fixing system wherein a toner image formed by an image forming means in an image forming apparatus is heated and fixed on a recording medium such as paper.
In a conventional fixing apparatus of this type, a heating roller as a first rotating member is used as a fixing roller. A pressure roller is used as a second rotating member which is brought into rolling contact with the first rotating member. This fixing system is called a heating roller fixing system.
However, several problems are presented in the conventional fixing apparatus of this type. For example, the characteristics of the finally fixed medium and the image quality are not acceptable. Many factors influence these problems of the fixing apparatus, and no fixing apparatus which satisfies requirements of all such factors has been proposed.
A thermal element concerning the fixing property is described in (a) U.S. Pat. No. 3,945,726 (similar descriptions are exemplified by Japanese Patent Publication No. 35498/1979 and Japanese Utility Model Publication No. 60020/1982). According to U.S. Pat. No. 3,945,726, a heat-insulating bush is inserted between a bearing of a frame member for supporting a heating roller and the heating roller. In (b) Japanese Utility Model Desclosure No. 145061/1981, as an application example, a heat-insulating material is inserted between a drive gear and a heat roller and a heat-insulating bush is mounted in a bearing so as to prevent heat conduction to the drive gear.
However, according to the invention (a), the heat loss is excessively large, and temperatures at end portions of the heat roller become low. In the utility model (b), a decrease in temperature at the central portion of the heating roller becomes large when sheets are continuously fixed, resulting in incomplete fixing at the central portion of the heating roller, although there is no problem in case of noncontinuous sheet feeding.
Unlike the prior art described above, an arrangement has been proposed wherein a temperature profile of the heater at the end portions is increased to obtain a uniform temperature profile along the axial direction of the heating roller so as to compensate for the heat loss. However, according to this arrangememt, the heat loss at the end portions of the heating roller is increased, and thus the heating efficiency is degraded. In addition to this disadvantage, an increase in temperature occurs inside the system using this fixing apparatus. Blocking of toner in the cleaner and the developing unit occurs, plastic components are thermally deformed, and other problems also occur. The above conventional techniques have been proposed to prevent the thermal loss and improve the fixing property. The technical level is associated with exothermic properties. As described above, the fixing roller is maintained in a thermally floating state. Then, the fixing roller is not electrically grounded since electrically insulating plastic members such as the heat-insulating sleeve, the rolling bearing or the heat-insulating member are used.
In this state, when the sheet passes between the pair of fixing rollers for fixing a toner image on the sheet, the roller is highly charged upon one fixing operation, resulting in a spark or discharge.
The present inventor found that degradation in image quality after fixing was caused by discharge of charges accumulated during fixing. The present inventor studied to develop a fixing apparatus capable of maintaining the fixing level at high level and for obtaining a clear fixed image and found that an erroneous operation (jam detection disable state, paper feed error, overrunning, no restarting, temperature control failure, etc.) is caused by the spark and discharge (these are assumed to occur by a spurious signal or a high current which may be supplied to the circuit of the image forming apparatus) and that proper electronic control cannot be performed if such an erroneous operation is caused.
In another conventional fixing apparatus described in Japanese Utility Model Disclosure No. 58136/1979, a conductive brush is brought into contact with the entire surface of an insulating elastic layer as a coating of the fixing roller surface so as to prevent the sheet from being wound therearound. However, according to this arrangement, a toner or paper dust is attached to the distal end of the brush, the discharge effect is greatly decreased, and an effect of preventing the sheet from being wound around the fixing roller cannot be sufficiently obtained. In addition, the roller surface is partially charged and discharged. Under this condition, spark and discharge occur between the conductive portion in the vicinity of a contaminated leading end thereof and a charged portion on the roller surface. This phenomenon is not associated with the technical level of Japanese Utility Model Disclosure No. 58136/1979, but becomes a critical issue for the present inventor.
No conventional fixing apparatus satisfies requirements for both high-speed fixing and clear image quality after fixing.